Large power transformers are generally cooled by oil which passes through the transformers to collect heat and then through a heat exchanger to release the heat. Catastrophic failures of large power transformers have been traced to electrification of the oil. An electric field is generated by the convection of oil through the windings, with the oil playing the role of a Van de Graaff generator belt. There is a major need for a device that can be used to provide an unambiguous measurement of the net charge entrained in the oil.
Prior to the present invention, attempts to provide absolute measurement of the charge entrained in the oil or other fluid have been unsuccessful. The attempts include the direct pickup of charge by ground as well as capacitive pickups. In the first type of sensor, where the measured quantity is a conduction current, even if measures are taken to control the fields induced by the net charge so that the measured current might be related to the absolute charge density entrained in the liquid, one can never be certain whether the net charge is causing the detected current or whether the detected current is in part or even wholly due to charge removed from the electrode by the flow. Techniques that make use of a large settling chamber, where there is presumably a sufficient residence time of the fluid for charge relaxation to reach completion, suffer from requiring an inconveniently large inventory of liquid and always leave open the question of whether or not the fluid leaving the settling chamber does so without carrying charge. This latter charge, which would make a misleading contribution to the measured current, could either result because the charge relaxation process had not reached completion or because of entrainment of charge in the chamber by the very electrification process to which the instrument is to be applied. In the second type of instrument, there is an inherent difficulty in distinguishing between capacitive and conduction currents. Any device actually in contact with the fluid will be subject to conduction and electrification currents. If the device is separated from the fluid by an insulating layer, there will always be the question of whether or not part of the charge being measured is due to the insulating layer and therefore artifactual.